A token filter of type porter_stem that transforms the token stream as
per the Porter stemming algorithm.

Note, the input to the stemming filter must already be in lower case, so
you will need to use
Lower Case Token Filter or
Lower Case Tokenizer farther down the Tokenizer chain in order for this to
work properly!. For example, when using custom analyzer, make sure the
lowercase filter comes before the porter_stem filter in the list of
filters.